Former 'Doctor Who' star David Tennant has reportedly joined the cast of the movie 'Mary Queen of Scots', which will star actress Saoirse Ronan as the titular character. The film, which is expected to go into production this year, is based on the true story of Mary's attempt to overthrow her cousin Elizabeth's seat on the English throne, which led to her being condemned to jail before facing execution. In the movie, Elizabeth I will be played by Margot Robbie. Tennant's role in the movie, which is expected to be released next year, is unknown as of this time.